Method for improving the coking resistance of a lubricating composition

ABSTRACT

Disclosed is a method of lubrication including a step of lubricating a machine with a lubricating composition, the lubricating composition including 20-100% ionic liquid or of a mixture of several ionic liquids. The ionic liquid or the mixture is selected from at least: an anion A −  chosen from sulfonylimides, the substituent(s) of which is (are) independently chosen from a fluoroalkyl, fluoroether, perfluorinated alkyl or perfluoroether group; and a cation C +  including a nitrogen-containing heterocycle or a quaternary ammonium, the substituent(s) of which is (are) independently chosen from: a hydrogen atom or alkyl, alkoxy, fluorinated alkyl, perfluorinated alkyl, alkylsilane, alkyl alcohol, vinyl, alkyl allyl, ether or polyether groups having a linear or branched chain having 1-3 carbon atoms. The deposition start temperature in thin film of the ionic liquid or of the mixture of ionic liquids is at least equal to 330° C. The lubricating composition reduces deposits formed in the machine.

FIELD OF THE INVENTION

The present invention relates to the field of lubricating compositions, especially for gas turbines of jet engines of aircraft or further for conveyor chains.

Especially, the present invention relates to a method of diminishing the thin-film coking, at very high temperatures, of a lubricating composition comprising one or a mixture of several specific ionic liquids, especially as a base oil.

PRIOR ART

The gas turbines of aircraft need lubricating compositions of high performance which can be used over a very wide range of temperatures. The lubricating compositions intended for civilian and military aviation have to comply with very precise specifications figuring in detail in standards such as MIL PRF 23699 set up by the military authorities or standard AS 5780 set up by the SAE International. These standards determine, among other features, the physical and chemical properties, the levels of thermal stability, the stability with respect to oxidation, or the anti-wear properties of the lubricating compositions.

During the last sixty years, the performances of aircraft and of their engines have increased very significantly, thereby creating new requirements for the lubricating compositions.

The new conditions for aviation, for military aviation as well as for civilian aviation, such as the drastic reduction of the emissions of atmospheric polluting matter such as the nitrogen oxides NOx, the reduction of noise and of the consumption of fuel need an important upgrade of the efficiency of the propulsion group of the aircraft, while seeking to reduce the mass of the equipment. That improvement has a direct influence on the temperature of the gases at the intake and at the outlet of the turbine. Indeed, the intake temperature of the air of the turbines did not stop increasing, going from 800° C. in the 1960s to 1700° C. in the 2000s. That increase goes along with an increase of the temperature of the lubricating compositions in the roller bearings of the turbines: close to 150° C. in the 1960s and now, in continuous operation, 220° C.

Thus, it is absolutely necessary that the lubricating composition for turbines be thermally stable, resistant to coking and to oxidation or thermal oxidation. Research to this end for good thermal properties, especially as far as coking is concerned, is a constant task since the beginning of development of aeronautic lubricants.

Indeed, the resistance to coking makes it possible to avoid forming of coal-type residues, called coke, which are harmful for a good operation of the turbine of an aircraft, such as blocking of the conduits of the oil circuit for the lubricating composition and/or clogging of the filters. Coking generally leads to a high reduction of the flow rate of the oil circulation in the cooling exchangers, resulting in overheating or in a defect of lubricating the roller bearings of the turbines of the engine.

Coke is essentially formed when the turbines are off: since the oil does not circulate any more, it drains off and leaves no more than a thin oil film on the metallic surfaces. And since they are not cooled any more, the accumulated calories are not evacuated any more, and therefore, there is locally a very high increase of the surface temperatures, a phenomenon which is known as “heat-soak back”.

The same problem of thermal resistance and of resistance to coking exists for conveyor chains.

Indeed, in certain industries such as, for example the production of panels made of wood particle chipboard or panels made of plaster, or in the glass industry, the temperatures of the ovens are higher and higher in order to increase the production speeds. They can reach or even go beyond 250° C. with possible peaks of 300° C. or even higher in the glass industry. The conventional lubricants, mineral or synthetic based,—such as the esters of trimellitic acid, which are largely used for that application, or the thermostable esters of neopolyols, mixed with the anti-oxidation or anti-coke additives used for the oils for turbines for aviation—reach the limits of their thermal stability and of their resistance to coking and form gums which coke over time under that temperature.

As for the oils for aircraft turbines, the crucial point is not so much a very high thermal resistance, which would result in high temperatures for the beginning of degradation during TGA measurements as there again, resistance to thin-film cokings. Indeed, the lubricant is generally applied to conveyor chains on a drop by drop basis or by spraying, which results in thin films of lubricant which are then exposed to very high temperatures when the chains pass into the ovens. Under those conditions, coke is formed when the lubricant is not sufficiently resistant, said coke accumulating until it blocks the chains with a subsequent stop of the production becoming necessary for maintenance and repair.

Therefore, the problem is essentially the same as the one of the oils for aircraft turbines described before. In both cases, it is essential to have products which are able to resist coking at high temperature.

The progress of the lubricants as far as both their base, which was initially mineral and then synthetic (mainly esters), and their additives and their chemical features are concerned, has up to now made it possible to respond to the increasing technical requirements of the turbines and to the evolution of the specifications. That continuous process of improvement has led to the present, state-of-the-art technology, including the use of esters of carboxylic acids of neopolyols, especially pentaerythritols, combined with anti-oxidizing products of the third generation (polymerized aromatic amines).

New solutions have also been proposed in the prior art.

Indeed, numerous types of thermostable chemical molecules have been studied in the years 1990 to 1995, at the instigation of the US Air Force, for replacing the esters of neopolyols, among which are the oils based on silicone, on perfluoropolyether (PFPE) or on polyphenylene ether (PPE).

However, none of said studied molecules or formulae has been satisfying.

Since the end of the 1970s and more exactly since the beginning of the 2000s, a new type of molecules, the ionic liquids (LI), have been used and developed for different applications among which are the lubricants.

They are used in lubricating compositions either as a lubricating base with or without adding additives or as an additive among others in a conventional base, or as a combination of both.

The patent EP-2,602,307 describes, for example, the use of ionic liquids as a base for lubricating compositions for applications at high temperature and under high vacuum and having good anti-rust properties thanks to the addition of an additive (long-chain amine salt). However, that document does not mention the problem of coking arising from the use of the lubricant.

The patent US-2009/0069204 describes a composition for roller oil the base of which can comprise 1 to 100 mass-%, preferably 50 to 100 mass-% of an ionic liquid. That document mentions an exhaustive list of cations and an exhaustive list of anions which might be suitable for forming the ionic liquid. The results of experimental testings show by thermogravimetric analysis (TGA) that the ionic liquids have a good thermal stability. Indeed, generally, the behavior of the ionic liquids at high temperature is determined by TGA.

It shall be noted that the TGA method is a technique of thermal analysis which consists in measuring the variation of the mass of a small sample (several milligrams) over time for a given temperature or a given temperature profile. It makes it possible, among other possibilities, to determine the temperature at which thermal degradation begins; the higher that temperature is, the more that product is resistant to high temperature.

However, as will be shown further down in the experimental part, a high temperature of beginning of thermal degradation determined for an ionic liquid by TGA (under an inert atmosphere) does not necessarily indicate that this liquid has a resistance to coking at high temperature, such as a resistance to coking at high temperature in thin film.

Document US 2013/053287 describes a lubricating composition intended for lubricating machines. In particular, the lubricating composition comprises:

-   -   (A) a fluorine atom-containing ionic liquid as main component,         and     -   (B) from 0.001 to 5 parts by weight per 100 parts by weight of         the ionic liquid (A) of an imidazolium phosphate ester salt.

Also known from the prior art is document US 2007/295478, which describes an apparatus for adjusting temperatures that comprises a compressor comprising moving parts that are lubricated by one or more ionic liquids. This document cites an exhaustive list of ionic liquids that may be suitable in order to lubricate the compressor.

Further, document WO-2011/026990 and also document US-2010/0227785 mention the use of ionic liquids as additives in conventional synthetic ester-based oil in order to reduce the accumulation of suspended deposits and the coke for the first document and the formation of deposits for the second document.

More specifically, document WO-2011/026990 describes a lubricating composition comprising, by mass, with respect to its total mass, from 50 to 99% of a conventional oil base such as a synthetic ester, from 0.01 to 5% and more specifically 0.2 mass-% of an ionic liquid C⁺A⁻ and from 0.01 to 10 mass-% of an additive. The cation C⁺ may be selected especially among the cations: imidazolium, pyridinium, pyrazolium, oxazolium, etc., and the anion may be selected especially among a fluorinated phosphate, a fluorinated borate, a perfluorinated acetate, a perfluorinated sulfonate, etc. It is indicated that the use of ionic liquid as an additive would make it possible to reduce the deposits in suspension and the formation of coke. In the experimental part, in view of showing that effect, two ionic liquids have been tested as an additive, at 0.2%, in a standard oil by a test HLPS (Liquid Process Simulator) according to SAE ARP 5996: methyl(trioctyl)ammonium trifluoroacetate and trihexyl-tetradecyl-phosphonium bis(trifluoromethylsulfonyl)imide. However, those ionic liquids, used as a base, do not make it possible to solve the problems of coking at high temperature as will be shown further down in the examples.

Document US-2010/0227785 describes a lubricating composition which is applied for oils for internal combustion engines, mainly for automobiles, and comprising 0.01 to 5 mass-% of an ionic liquid. That document also mentions exhaustive lists of cations and of anions which might be suitable for the present invention. The lubricating composition comprises preferably 4-methyl-butylpyridinium tetrafluoroborate or 4-methyl-butylpyridinium hexafluorophoshate. The preparation further comprises zinc dialkyl dithiophosphate (ZDDP), or molybdenum dialkyl dithiophosphate, or zinc dialkyl dithiocarbamate, or molybdenum dialkyl dithiocarbamate (MoDTC) in a ratio of 1:10 to 10:1 with respect to the ionic liquid. The reduction of deposits is characterized according to ASTM D7097, <<Moderately High Temperature Thermo-Oxidation Engine Test>> (TEOST MHT) where the deposits are measured at the end of the test on a heated tube at 285° C. with repeated passing of an oil comprising a catalyst as additive, under air during 24 hours. However, according to that document, the ionic liquids which make it possible to reduce the deposits when they are used as additive in a conventional oil, cannot be used as a base since they are not resistant to oxidation and to the formation of deposits.

Therefore, there is a need in prior art for new compounds for use as lubricants, especially at high temperature, for machines such as turbines, for example turbines for aircraft, or conveyor chains, and which shall be alternatives or improvements with respect to known compounds.

Thus, the invention aims at proposing a new method of reducing the deposits in a machine such as a turbine, lubricated with a lubricating composition which avoids at least part of the inconveniences detailed above and making it possible particularly to improve their resistance to thin-film coking at high temperature under critical real conditions.

SUMMARY OF THE INVENTION

The present invention is thus directed to a method of lubricating which comprises a step of lubricating a machine such as an engine, a turbine, a conveyor chain with a lubricating composition, the lubricating composition comprising, in mass with respect to the total mass of said lubricating composition, from 20% to 100% of an ionic liquid or of a mixture of several ionic liquids,

characterized in that said ionic liquid or said mixture of ionic liquids is selected from an ionic liquid or a mixture of ionic liquids comprising at least:

-   -   an anion A⁻ chosen from sulfonylimides, the substituent(s) of         which is (are) independently chosen from a fluoroalkyl,         fluoroether, perfluorinated alkyl or perfluoroether group, and     -   a cation C⁺ comprising a nitrogen-containing heterocycle or a         quaternary ammonium, the substituent(s) of which is (are)         independently chosen from: a hydrogen atom or alkyl, alkoxy,         fluorinated alkyl, perfluorinated alkyl, alkylsilane, alkyl         alcohol, vinyl, alkyl allyl, ether or polyether groups having a         linear or branched chain having from 1 to 3 carbon atoms, on         condition that when the cation is a quaternary ammonium, at         least two of these substituents are a methyl group,

the deposition start temperature (TDD) in thin film, determined by the MCT method according to the standard GFC Lu-27 A-13, of the ionic liquid or of the mixture of ionic liquids being at least equal to 330° C.,

said lubricating composition being suitable to reduce the deposits, such as carbonaceous residues, formed in said machine.

Within the present invention, an ionic liquid is a chemical compound composed of an anion A⁻ and a cation C⁺, said substance having a melting point lower than 100° C.

In the present invention, the way of measuring thermal stability is determined by the MCT method (Micro Coking Test) according to the standard GFC Lu-27 A-13, version 2 (formerly GFC Lu-27-T-07). The MCT is an experimental testing which makes it possible to evaluate the tendency of a lubricating composition to form deposits when it is under high temperatures (thermal stability) as well as to estimate the behavior of a lubricating composition during a test in an engine.

The applicant has selected MCT as a test for characterizing the resistance to coking, since it is an appropriate, representative, and believable test. It distinguishes perfectly, for example, the oils for aviation available on the market, showing the superiority, as to thermal resistance, of the oils MIL-PRF 23699 of HTS grade (High Thermal Stability) versus oils of the Standard grade, as will be shown further down in Example E. It is by the way a relatively quick test consuming small amounts of the product.

The test conditions are the following ones:

-   -   0.6 ml of oil without any anti-foam additive (adding an         anti-foam additive has been generalized for that method for         present oils; however, the anti-foam additives are not         compatible with most of the ionic liquids, since they lead to         premature thermal degradation);     -   duration: 90 min;     -   plate made of an aluminum alloy, tilted by 1.5% +/−0.05% toward         the hot point which is situated at the lower point, and         comprising a trough;     -   temperature gradient generally from 230° C. to 280° C. and in         the present case from 250° C. to 330° C. or more (using a         temperature gradient above 300° C. makes it possible to         determine precisely the TDD which are superior to that         temperature, such as using the gradient 280° C. to 350° C.).

When the test conditions are fulfilled, a quantity of 0.6 ml of the lubricating composition to be tested is placed into the trough of a plate made of an aluminum alloy (see FIG. 1 which illustrates a plate according to this test) and is exposed to a temperature gradient of 250° C. to 330° C., or even from 250° C. to 350° C.

After 90 minutes of heating, the excess oil, which is still liquid, is withdrawn, drained, and rinsed with a mineral oil, then the plate is left to cool before being degreased. Then, the place and the aspect of the deposits are evaluated for determining the deposition start temperature.

For doing this, the color of the deposits obtained on the trough is compared to the standard varnishes as defined according to the CRC varnish scale—scale C. The deposition start temperature corresponds to an AMF of 9 (mean coefficient of merit obtained for a plate of essentially intrinsic color to slightly colored). By assimilating the gradient to a thermal straight-line, that temperature is determined by:

TDD=θ1−[(θ1−θ2)×L/81] with:

-   -   TDD=deposition start temperature of size AMF 9 (° C.),     -   θ1=adjustment temperature of the hot point (° C.),     -   θ2=adjustment temperature of the cold point (° C.),     -   L=distance between the adjusting hole and the beginning of         deposits of size AMF 9 (mm),     -   81=distance (mm) between the two holes for thermal probes.

Thus, with MCT, the higher the value of the deposition start temperature (TDD), the better is the thermal stability of the lubricating composition and consequently the better the resistance to coking.

Further, within the present invention, the term “machine” designates all types of mechanical device that can be lubricated with a lubricating composition, especially in order to improve its functioning. Such a machine can correspond for example to an engine, a turbine or further to a conveyor chain, especially a turbine or a conveyor chain working under high temperatures, such as between 200° C. and 500° C., preferably between 280° C. and 500° C.

DESCRIPTION OF THE INVENTION

The applicant strived to develop new compounds having an excellent resistance to thin-film coking at very high temperature (for example between 200° C. and 500° C.), suitable for a use in lubricating compositions, especially for turbines for aviation or for conveyor chains.

The applicant has further shown that a specific selection of cations and of anions forming an ionic liquid makes it possible to form a lubricating base for lubricating compositions used, for example, in turbine engines of aircraft.

Although it is known that ionic liquids have a good thermal stability (up to 500° C., determined by a thermogravimetric analysis, TGA), the applicant has surprisingly found that a particular combination of anions and of cations form ionic liquids having a very high resistance to thin-film coking, especially determined by the MCT method (standard GFC Lu-27 A-13, version 2) and that this surprising effect was not at all related to their thermal stability determined by the thermogravimetric analysis (TGA). In other words, even when an ionic liquid is known for having a good thermal stability by TGA, this does not mean that it also has a good resistance to thin-film coking at high temperature, and especially when this is determined by a MicroCoking Test (MCT).

The applicant has thus further shown that the selected combination has improved properties (i.e. an improved resistance to thin-film coking) with respect to other ionic liquids and with respect to lubricating bases conventionally used especially for aircraft turbines or conveyor chains.

The applicant has shown that thermal stability as well as resistance to thin-film coking of an ionic liquid depends on the nature of the anion, on the nature of the cation and on their substituents.

Different publications describe that alkyl chains having more than six atoms of carbon on the cationic part of the ionic liquid make it possible to increase the lubricating performance of the ionic liquids.

The applicant has however surprisingly discovered that cations having substituents with short chains, such as an alkyl chain, alkyl silane, alcohol, or a C1-C3 alkoxy chain, make it possible to improve the resistance to thin-film coking and to obtain a TDD of at least 330° C. and preferably of at least 350° C., while having a good lubricating power.

The applicant has also discovered that the choice of the anion is not arbitrary and that only some anions selected among sulfonylimides make it possible to form, together with the appropriate cations, an ionic liquid having an excellent resistance to coking. That effect can be further improved by a particular selection of the substituents of the anion, which are preferably substituents selected among: fluoroalkyl, fluoroether, perfluorinated alkyl or perfluoroethers.

The present invention therefore aims at a method of lubrication which comprises a step of lubricating a machine, such as an engine or a turbine, with a lubricating composition, the lubricating composition comprising, in mass with respect to the total mass of said lubricating composition, from 20% to 100% of an ionic liquid or of a mixture of several ionic liquids,

characterized in that said ionic liquid or said mixture of ionic liquids is selected from an ionic liquid or a mixture of ionic liquids comprising at least:

-   -   an anion A⁻ chosen from sulfonylimides, the substituent(s) of         which is (are) independently chosen from a fluoroalkyl,         fluoroether, perfluorinated alkyl or perfluoroether group, and     -   a cation C⁺ comprising a nitrogen-containing heterocycle or a         quaternary ammonium, the substituent(s) of which is (are)         independently chosen from: a hydrogen atom or alkyl, alkoxy,         fluorinated alkyl, perfluorinated alkyl, alkylsilane, alkyl         alcohol, vinyl, alkyl allyl, ether or polyether groups having a         linear or branched chain having from 1 to 3 carbon atoms, on         condition that when the cation is a quaternary ammonium, at         least two of these substituents are a methyl group,

the deposition start temperature (TDD) in thin film, determined by the MCT method according to the standard GFC Lu-27 A-13, of the ionic liquid or of the mixture of ionic liquids being at least equal to 330° C.,

said lubricating composition being suitable to reduce the deposits, such as carbonaceous residues, formed in said machine.

Another subject of the present invention is the use of a lubricating composition for reducing the deposits, such as carbonaceous residues, formed in a machine (for example during a lubricating step), in which said lubricating composition comprises, by mass with respect to its total mass, from 20% to 100% of an ionic liquid or of a mixture of several ionic liquids, said ionic liquid or said mixture of ionic liquids being selected from an ionic liquid or a mixture of ionic liquids comprising at least:

-   -   an anion A⁻ chosen from sulfonylimides, the substituent(s) of         which is (are) independently chosen from a fluoroalkyl,         fluoroether, perfluorinated alkyl or perfluoroether group, and     -   a cation C⁺ comprising a nitrogen-containing heterocycle or a         quaternary ammonium, the substituent(s) of which is (are)         independently chosen from: a hydrogen atom or alkyl, alkoxy,         fluorinated alkyl, perfluorinated alkyl, alkylsilane, alkyl         alcohol, vinyl, alkyl allyl, ether or polyether groups having a         linear or branched chain having from 1 to 3 carbon atoms, on         condition that when the cation is a quaternary ammonium, at         least two of these substituents are a methyl group,

the deposition start temperature (TDD) in thin film, determined by the MCT method according to the standard GFC Lu-27 A-13, of the ionic liquid or of the mixture of ionic liquids is at least equal to 330° C., preferably higher than or equal to 350° C.

For the remainder of the description below, the features are valid both for the lubricating process and for the use, described above.

Especially, the deposition start temperature TDD in thin film of said ionic liquid or of said mixture of ionic liquids, as determined by the MCT method, is higher than or equal to 330° C., preferably higher than or equal to 340° C., especially higher than 350° C.

According to the invention, a temperature TDD of at least 330° C. comprises especially the following values: 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340, 341, 342, 343, 344, 345, 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357, 358, 359, 360, 365, 370, 380, 385, 390, 395, 400, 405, 410, 415, 420, 425, 430, 435, 440, 445, 450, 455, 460, 465, 470, 475, 480, 485, 490, 495, 500, etc., or any interval situated between these values.

Thus, the ionic liquid or liquids selected according to the invention have an excellent thermal stability such that the lubricating step can be carried out under high temperatures of between 200° C. and 500° C., preferably between 280° C. and 500° C.

Thus, within the present invention, at least one cation C⁺ may be selected among the following ones, provided that it forms with the anion an ionic liquid having a TDD higher than or equal to 330° C.: a nitrogen-containing heterocycle or a quaternary ammonium.

Generally, the nitrogen-containing heterocycle according to the invention is selected among: imidazolium, pyrazolium, quinolium, pyridinium, piperidinium, oxazolium, thiazolium, benzothiazolium, morpholinium or one of the derivatives thereof. The term “derivatives” designates the derivatives of said cations according to the position of their hetero-atoms and the saturation of their cycle.

Generally, the nitrogen-containing heterocycle forming the cation C+ of the ionic liquid or of the mixture of ionic liquids according to the invention can be selected among: imidazolium, pyrazolium, quinolium, pyridinium, piperidinium, oxazolium, thiazolium, benzothiazolium or morpholinium or one of the derivatives thereof,

the substituent or substituents thereof being selected independently among: the hydrogen atom or the alkyl, aryl, aryloxy, alkylthioether, fluorinated alkyl, perfluorinated alkyl, alkyl silane, alkyl alcohol, allyl, vinyl, ether, arylether, arylthioether or polyether groups, having a linear or branched chain having from 1 to 3 carbon atoms according to the nature of the cation.

Within the present invention, it is understood that a linear chain or a branched chain having from 1 to 3 carbon atoms is a chain having a number of carbon atom(s) equal to 1, 2 or 3; these numbers comprising thus the intervals of 1 to 3, of 1 to 2 or of 2 to 3 carbon atoms.

The cation C⁺ according to the invention is particularly represented hereafter:

  imidazolium

  pyridinium

  piperidinium

  oxazolium

  benzothiazolium

  thiazolium

  pyrazolium

  morpholinium

  quinolium

  quatemary ammonium

whose substituents R₁ to R₁₂, such as indicated hereabove are identical or different and are selected independently among:

-   -   hydrogen atom and/or     -   alkyl, aryl, alkylthioether, arylether, arylthioether, aryloxy,         fluorinated alkyl, perfluorinated alkyl, alkyl silane, alkyl         alcohol, allyl, ether, arylether, arylthioether, or polyether         group with a linear chain or a branched chain having 1 to 3         carbon atoms.

Preferably, the nitrogen-containing heterocycle is selected among imidazolium, pyridinium, or pyrazolium.

As an example, the cation C⁺ can be an imidazolium cation having the above represented formula, i.e. :

where

R₄ and R₅ are identical or different, preferably hydrogen atoms;

R₁ and R₂ can be: a hydrogen atom, a methyl, ethyl, propyl, vinyl, allyl, cyano group, preferably a methyl group;

R₃ is independently selected among the following group: alkyl, alkyl silane, a fluorinated alkyl, a perfluorinated alkyl, an alkyl alcohol, an allyl, ether or polyether, said group being linear or branched having from 1 to 3 carbon atoms.

In particular, the imidazolium comprises at least two methyl groups at position 1 and 2 or at position 2 and 3.

Preferably, the imidazolium comprising a methyl group at position 1 and 2 or at position 2 and 3 comprises a hydrogen atom at position 4 and 5 and is preferably 1-ethyl-2,3-dimethyl-imidazolium or 1,2-dimethyl-3-((trimethylsilyl)methyl)imidazolium.

Even more preferably, when the substituents in position 1 and 2 of the imidazolium are methyls, the substituents in position 4 and 5 are hydrogen atoms and the substituent in position 3 is independently selected among the alkyl, fluorinated alkyl, perfluorinated alkyl, alkyl silane, alkyl alcohol or vinyl groups, having a linear chain or a branched chain with 1 to 3 carbon atoms.

According to a particular embodiment, the cation according to the invention is not 1-ethyl-3-methyl-imidazolium. According to this mode, the following ionic liquid can be excluded from the ionic liquids according to the invention: 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.

According to another embodiment, the cation can be a thiazolidinium with the formula represented above, i.e.:

where

R₃ and R₄ are identical or different, preferably hydrogen atoms,

R₁ may be a hydrogen, a methyl, ethyl, propyl, vinyl, allyl, cyano group; preferably a methyl group,

R₂ is independently selected among the following group: an alkyl, a fluorinated alkyl, a perfluorinated alkyl, an alkyl alcohol, an allyl, a vinyl, an ether or a polyether, said group being linear or branched with 1 to 3 carbon atoms.

As well, the cation C+ can be a pyridinium cation having the formula represented above, i.e.:

where

R₁ is independently selected among: an alkyl, fluorinated alkyl, perfluorinated alkyl, alkyl silane, alkyl alcohol, allyl, vinyl, ether or polyether group, preferably among an alkyl, alkyl silane, ether, fluorinated alkyl, perfluorinated alkyl group, said group being linear or branched and having 1 to 3 carbon atoms;

R₂ to R₆ are identical or different, selected among: hydrogen atom, methyl group, ethyl group, a halogenated derivative, a dimethyl amino group, preferably hydrogen atoms.

In particular, the pyridinium comprises at least one methyl group, or even two, preferably at position 3, 4 or 5.

Preferably, the pyridinium comprises at least two methyl groups at position 3 and 5.

The cation C⁺ can be a morpholinium cation with the formula represented above, i.e.:

where

R₁ to R₈ are identical or different, selected among: hydrogen atom, a methyl or ethyl group, preferably hydrogen atoms;

R₉ and R₁₀ are independently selected among an alkyl, fluorinated alkyl, perfluorinated alkyl, alkyl silane, alkyl alcohol, allyl, vinyl, ether or polyether group, preferably among an alkyl, alkyl silane, ether, alkyl alcohol, fluorinated alkyl, perfluorinated alkyl group, said group being linear or branched with 1 to 3 carbon atoms; R₉ is preferably a methyl.

The cation C⁺ can be a piperidinium cation having the formula represented above, i.e.:

where

R₁ to R₁₀ are identical or different, selected among: hydrogen atom, a methyl or ethyl group, preferably a hydrogen atom;

R₁₁ and R₁₂ are independently selected among an alkyl, fluorinated alkyl, perfluorinated alkyl, alkyl silane, alkyl alcohol, allyl, vinyl, ether or polyether group; preferably among an alkyl, alkyl silane, ether, fluorinated alkyl, perfluorinated alkyl group, said group being linear or branched with 1 to 3 carbon atoms; R₁₁ is preferably a methyl group.

Equally, the cation C⁺ can be a quaternary ammonium cation having the formula represented above, i.e.:

where

R₁, R₂, R₃ and R₄ are identical or different, are selected among an alkyl, fluorinated alkyl, perfluorinated alkyl, alkyl silane, alkyl alcohol, ether or polyether group with a linear chain or a branched chain with 1 to 3, on condition that at least two among the radicals R₁ to R₄ are methyl groups.

As an example, the cation C⁺ can be a quinolium cation having the formula such as represented further up, i.e.:

where

R₂, R₃, R₄, R₅, R₆, R₇ and R₈ are identical or different, selected among: hydrogen atom, a methyl, ethyl, propyl, butyl, dimethylamine group, preferably hydrogen atoms;

R₁ is independently selected among: an alkyl, fluorinated alkyl, perfluorinated alkyl, an alkyl alcohol, an ether or a polyether, said group being linear or branched with 1 to 3 carbon atoms.

As an example, the cation C⁺ can be a pyrazolium cation with the formula such as represented further up, i.e.:

where

R₂ and R₃ are identical or different, preferably hydrogen atoms;

R₁ and R₅ may be: hydrogen atom, a methyl, ethyl, propyl, vinyl, cyano group, preferably a methyl group;

R₄ is independently selected among: an alkyl, alkyl silane, fluorinated alkyl, perfluorinated alkyl, alkyl alcohol, an ether or a polyether group, said group being linear or branched with 1 to 3 carbon atoms.

As a further but not limiting example, the substituents R₁ to R₁₂ of the cations according to the invention are groups or linear or branched chains which can be selected among the following ones, provided the conditions listed further up are fulfilled:

the alkyls: methyl, ethyl, propyl, butyl;

the fluorinated alkyls: trifluoromethyl, trifluoroethyl, trifluoropropyl, etc;

the perfluorinated alkyls: perfluoromethyl, perfluoroethyl, perfluoropropyl, etc;

the alkyl silanes: trimethylsilylmethyl; triethylsilylmethyl; trimethylsilylethyl; trimethylsilylpropyl; etc;

the alkyl alcohols: hydroxymethyl, hydroxyethyl, hydroxypropyl, etc

the alkenes: vinyl, allyl, etc;

the ethers: methoxymethyl, methoxyethyl, methoxypropyl, ethoxymethyl, etc;

the polyethers: polymethylene, polyethylene, polypropylene, etc.

Generally, all of the radicals R₁ to R₁₂ have short chains and comprise no more than 3 carbon atoms.

The applicant has further discovered that the selection of the anion and its substituents is important for forming an ionic liquid having a good resistance to coking for thin films.

As mentioned above, the anion A⁻ according to the invention is selected among sulfonylimide, whose substituent or substituents are independently selected among the following: a fluoroalkyl, fluoroether, perfluorinated alkyl or perfluoroether group.

In particular, the sulfonylimide compound according to the invention corresponds to the following general formula:

where R₁ and R₂ are identical or different, independently selected among a fluoroalkyl, fluoroether, perfluorinated alkyl or perfluoroether group.

As an example, the following compounds are suitable as sulfonylimide according to the invention: [(CF₃SO₂)₂N]⁻, [(CF₃CF₂SO₂)₂N]⁻, [(CF₃CF₂CF₂CF₂SO₂)₂N]⁻ or [(CF₃CF₂CF₂SO₂)₂N]⁻.

Generally, the ionic liquids or at least one of the ionic liquids of the mixture being suitable within the terms of the present invention can be selected among:

-   -   3-(2-hydroxyethyl)-2,3-dimethylimidazolium         bis(trifluoromethylsulfonyl)imide;     -   3-(2-hydroxypropyl)-2,3-dimethylimidazolium         bis(trifluoromethylsulfonyl)imide;     -   1-ethyl-2,3-dimethylimidazolium         bis(trifluoromethylsulfonyl)imide;     -   1-ethyl-2,3-dimethylimidazolium         bis(pentafluoroethylsulfonyl)imide;     -   3-allyl-1,2-dimethylimidazolium         bis(trifluoromethylsulfonyl)imide;     -   1,2-dimethyl-3-(trimethylsilyl)methyl)imidazolium         bis(pentafluoroethylsulfonyl)-imide;     -   1,2-dimethyl-3-((trimethylsilyl)methyl)imidazolium         bis(trifluoromethylsulfonyl)-imide;     -   1,2-dimethyl-3-((trimethylsilyl)propyl)imidazolium         bis(trifluoromethylsulfonyl)-imide;     -   2,3-dimethyl-1-propylimidazolium         bis(pentafluoroethylsulfonyl)imide;     -   2,3-dimethyl-1-propylimidazolium         bis(trifluoromethylsulfonyl)imide;     -   2,3-dimethyl-1-propylimidazolium         1,1,2,2,3,3-hexafluoropropane-1,3-disulfonyl-imide;     -   3-(2-methoxyethyl)-1,2-dimethylimidazolium         bis(pentafluoroethylsulfonyl)imide;     -   3-(2-methoxyethyl)-1,2-dimethylimidazolium         bis(trifluoromethylsulfonyl)imide;     -   2,3-dimethyl-1-propanolimidazolium         bis(trifluoromethylsulfonyl)imide;     -   1,2 dimethyl-3-allylimidazolium         bis(trifluoroethylsulfonyl)imide;     -   1-propyl-3,5-dimethylpyridinium         bis(trifluoromethyl)sulfonyl)imide;     -   1-(2-hydroxyethyl)-3,5-dimethylpyridinium         bis(trifluoromethyl)sulfonyl)imide;     -   ethyldimethylpropylammonium bis(trifluoromethylsulfonyl)imide;     -   N-ethyl-2-hydroxy-N,N-dimethylethaneammonium         bis(trifluoromethylsulfonyl)-imide;     -   N-ethyl-2-hydroxy-N,N-dimethylethaneammonium         bis(pentafluoroethylsulfonyl)-imide;     -   N-(2-hydroxyethyl)-N,N-dimethylpropaneammonium         bis(trifluoromethylsulfonyl)-imide;     -   1-propyl-4-methylpyridin-1-ium         bis(trifluoromethylsulfonyl)imide;     -   1-ethyl-3-methylpyridin-1-ium bis(trifluoromethylsulfonyl)imide;     -   1-propyl-3,5-dimethylpyridinium         bis(trifluoromethylsulfonyl)imide;     -   1-(2-hydroxyethyl)-3,5-dimethylpyridinium         bis(trifluoromethylsulfonyl)imide;     -   or a mixture thereof.

As an example, the mixtures of the following ionic liquids are suitable within the present invention and make it possible to have a TDD≧330° C.:

-   -   80% 1-propyl-2,3-dimethylimidazolium         bis(trifluoromethylsulfonyl)imide and 20%         1,2-dimethyl-3-(trimethylsilyl)imidazolium         bis(pentafluoromethylsulfonyl);     -   80% 1-propyl-2,3-dimethylimidazolium         bis(trifluoromethylsulfonyl)imide and 20%         1,2-dimethyl-3-(trimethylsilyl)imidazolium         bis(trifluoromethylsulfonyl)imide;     -   80% 1-propyl-2,3-dimethylimidazolium         bis(trifluoromethylsulfonyl)imide and 20%         1,2-dimethyl-3-ethylimidazolium         bis(trifluoromethylsulfonyl)imide.

Preferably, the ionic liquid or the mixture of ionic liquids is chosen from:

-   -   1-ethyl-2,3-dimethylimidazolium         bis(trifluoromethylsulfonyl)imide;     -   1,2-dimethyl-3-((trimethylsilyl)methyl)imidazolium         bis(trifluoromethylsulfonyl)-imide;     -   1,2-dimethyl-3-((trimethylsilyl)methyl)imidazolium         bis(pentafluoromethyl-sulfonyl)imide.

As indicated here above, the lubricating composition comprises in mass, with respect to its total mass, from 20% to 100% ionic liquid or ionic liquids, preferably from 50% to 100% and ideally from 75% to 100%.

Within the present invention, the term “at least 20 mass-%” comprises at least 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 92%, 95%, 97%, 98%, 99%, 99.5%, 99.9%.

Generally, the ionic liquid(s) constitute(s) the lubricating base of the lubricating composition.

According to one feature of the invention, that lubricating base can also comprise conventional oils known to the person skilled in the art, such as one or more long-chain esters.

Advantageously, the ester(s) with long chains is/are selected among the group formed by the reaction products of one or more polyols with one or more carboxylic acid(s) having 4 to 12 carbon atoms, the carbon chains of said carboxylic acids being either linear or branched. For a non limitative illustration purpose only, polyols appropriate for obtaining esters suitable for use in lubricating compositions for aircraft turbines include trimethylol propane, pentaerythritol, dipentaerythritol, neopentylglycol, tripentaerythritol, ditrimethylol propane, and their mixtures. For a non limitative illustration purpose only, carboxylic acids appropriate for obtaining esters suitable for use in lubricating compositions for aircraft turbines include: valeric acid, isovaleric acid, heptanoic acid, caprylic acid, nonanoic acid, isononanoic acid and capric acid. Thus, the long-chain ester(s) of said lubricating composition may be selected from products resulting from the reaction of pentaerythritol and of dipentaerythritol with one or more carboxylic acid(s) selected from the group consisting of valeric acid, isovaleric acid, heptanoic acid, caprylic acid, nonanoic acid, isononanoic acid and capric acid.

For a non limitative illustration purpose only, long-chain esters may be prepared by reacting a commercially available technical pentaerythritol with a mixture of carboxylic acids having 4 to 12 carbon atoms under standard esterification conditions that are well known to the person skilled in the art. Technical pentaerythritol is a mixture comprising from about 85% to 92% by weight of monopentaerythritol and from 8% to 15% by weight of dipentaerythritol. It may further comprise some amount of tri- and tetra-pentaerythritol which are traditionally formed as by-products during the preparation of technical pentaerythritol. As an example of a mixture of esters appropriate for the lubricating composition according to the invention, mention may be made of the composition of synthetic esters marketed by NYCO under the reference Nycobase 5750.

The lubricating composition comprises generally from 0 mass-% to 25 mass-% of one or more additives not belonging to the family of ionic liquids.

These additives are for example anti-wear agent(s), anti-corrosion agent(s) and/or yellow metal deactivating additive(s), antioxidant(s), or mixtures of two or more of these additives.

The anti-wear additives are known to the person skilled in the art and comprise among others the family of triarylphosphates (tricresyl phosphate, tri(butylphenyl) phosphate, tri(isopropylphenyl) phosphate, trixylylphosphate).

The anti-corrosion additive or additives and/or yellow metal deactivating additive(s) are selected from agents well known to the person skilled in the art, especially from derivatives of benzotriazole. For a non limitative illustration purpose only, particularly preferred additives include benzotriazole and methyl benzotriazole.

The antioxidant(s) in the lubricating composition of the invention may be selected from compounds well known to the person skilled in the art such as aromatic amines, aromatic amine oligomers, and their mixtures, that list being not limitative. In a preferred embodiment, the antioxidant agent(s) is or are selected from aromatic amines and, especially, from diaryl amines, N-arylnaphthyl amines, homo- and hetero-oligomers thereof and their mixtures. Aromatic rings of diaryl amines, N-arylnaphthyl amines and oligomers thereof may be optionally substituted with one or more alkyl group(s) comprising 2 to 10 carbon atoms. The person skilled in the art could for example refer to the international application WO95/16765, which discloses the preparation of an anti-oxidizing composition comprising diaryl amine oligomers as well as diaryl amine/N-aryl naphthyl amine heterodimers, or to the U.S. Pat. No. 5,489,711 which discloses the preparation of diaryl amine oligomers possessing anti-oxidizing properties.

For a non limitative illustration purpose only, particularly preferred antioxidants include di(octylphenyl)amines, octylphenyl-α-naphthyl amines and their oligomers.

The lubricating composition may comprise other additives known to the person skilled in the art, such as polymers which improve viscosity, dispersants, additives which lower the pour-point, etc.

The ionic liquids having a deterioration start temperature, as determined by MCT, of higher than or equal to 294° C. are preferably used for reducing the deposits formed in the machine when it is lubricated with the lubricating composition described above.

The lubricating composition as defined above may thus be used in machines for reducing the deposits of coke for thin films.

The examples hereinafter, which are not limiting, illustrate the present invention.

EXAMPLES

The applicant has shown by the following experimental testings that a precise combination of anions and cations as well as of their substituents make it possible to form an ionic liquid or a mixture of ionic liquids having an excellent resistance to coking for thin films.

The applicant has also tested the ionic liquids disclosed in the prior art cited further up. In a surprising manner, it has been stated that most of the ionic liquids as thin films degrade very quickly as the tables hereinafter show, notwithstanding their remarkable thermal stability which goes along with their very good results in TGA.

A. Experimental Protocol

The conditions of the experimental testing are as follows:

-   -   0.6 ml of oil without anti-foam     -   duration: 90 min     -   plate made of an aluminum alloy tilted by 1.5%+/−0.05% toward         the hot point which is situated at the lower point comprising a         trough     -   temperature gradient here generally from 250° C. to 300° C. (or         more, for example 280° C. to 350° C.).

When the temperature conditions are fulfilled, 0.6 ml of a lubricating composition to be tested is placed in the trough of a plate made of an aluminum alloy and is exposed to a temperature gradient of 250° C. to 330° C. (or more).

After 90 minutes of heating, the excess oil which is still liquid is withdrawn, drained and rinsed with a mineral oil. Then the plate is left to cool before being degreased. Then, the place and the aspect of the deposits are evaluated for determining the deposition start temperature.

For doing this, the color of the deposits obtained on the trough is compared to that of standard varnishes according to the CRC scale of varnishes—scale C. The deposition start temperature corresponds to an AMF of 9 (mean coefficient of merit obtained for a plate of essentially intrinsic color to slightly colored). By assimilating the gradient to a thermal straight line, that temperature is determined by (cf. FIG. 1 which represents diagrammatically a plate of that experimental testing):

TDD=θ1−[(θ1−θ2)×L/81]

with:

-   -   TDD=deposition start temperature of size AMF 9 (° C.),     -   θ1=adjustment temperature of the hot point (° C.),     -   θ2=adjustment temperature of the cold point (° C.),     -   L=distance between the adjusting hole and the beginning of         deposits of size AMF 9 (mm),     -   81=distance (mm) between the two holes for thermal probes.

Thus, with MCT, the higher the value of the deposition start temperature (TDD), the better is the thermal stability of the lubricating composition.

The TGA measurement was carried out with a temperature ramp of 10° C./min under an inert atmosphere (N₂ gas).

B. Influence of the Nature of the Anion on the Thermal Stability Determined by TGA and MCT of the Ionic Liquids (Table 1)

Table 1 shows that the nature of the anion is very important for the thermal stability of the ionic liquid.

These tests thus show that the anions A− chosen from sulfates, bis(fluorosulfonyl)imide, phosphates, dicyanamides, borates, sulfonates, thiolates or else acetates did not make it possible to obtain a deposition start temperature of at least 330° C., even if these compounds have a very good thermal stability according to the TGA test under N₂.

On the other hand, generally, whatever the nature of the cation as defined according to the invention, the perfluorosulfonylimide anions make it possible to have a deterioration start temperature as determined by MCT which is higher than or equal to 330° C. or even higher than or equal to 350° C.

C. Influence of the Nature of the Cation and of Its Substituents on the Thermal Stability Determined by TGA (N₂) and MCT of the Ionic Liquids (Table 2)

Consequently, the experimental testings show that particular cations according to the invention make it possible to obtain, combined with the appropriate anion, an ionic liquid having an excellent resistance to thin-film coking.

Further, as shown by examples 25 to 31, it is essential that the cations comprise the substituents according to the invention, i.e. hydrogen atom or alkyl, fluorinated alkyl, perfluorinated alkyl, alkyl silane, alkyl alcohol, allyl, ether or polyether group having a linear chain or a branched chain with 1 to 3 carbon atoms. In particular, when the cation is a quarternary ammonium, it is important that at least two of its substituents are a methyl as shown by examples 24 to 26 above.

D. Examples of Ionic Liquids Having a Degradation Start Temperature Determined by MCT Higher Than or Equal to 330° C. (Table 3).

TABLE 3 TGA MCT Ex Nomenclature Cation Anion (10° C./mn) (250-300° C.) 32 3-(2-hydroxyethyl)-2,3-dimethyl- imidazolium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]⁻ nd >330 33 3-(2-hydroxypropyl)-2,3-dimethyl- imidazolium bis(trifluoromethylsulfonyl)imide

436 >330 34 1-ethyl-2,3-dimethyl-imidazolium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]⁻ 457 >330 35 1-ethyl-2,3-dimethyl-imidazolium [(CF₃CF₂SO₂)₂N]⁻ 415 >330 bis(pentafluoroethylsulfonyl)imide 36 3-allyl-1,2-dimethyl-imidazolium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]⁻ 431 >330 37 1,2-dimethyl-3-((trimethylsilyl)methyl)- imidazolium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]⁻ 439 >330 38 1,2-dimethyl-3-((trimethylsilyl)methyl)- [(CF₃CF₂SO₂)₂N]⁻ 412 >330 imidazolium bis(pentafluoroethylsulfonyl)imide 39 1,2-dimethyl-3-((trimethylsilyl)propyl)- imidazolium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]⁻ 401 >330 40 2,3-dimethyl-1-propyl-imidazolium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]⁻ 459 >330 41 2,3-dimethyl-1-propyl-imidazolium [(CF₃CF₂SO₂)₂N]⁻ 412 >330 bis(pentafluoroethylsulfonyl)imide 42 2,3-dimethyl-1-propyl-imidazolium [(CF₃CF₂CF₂CF₂SO₂)₂N]⁻ nd >330 bis(nonafluorobutanesulfonyl)imide 43 2,3-dimethyl-1-propyl-imidazolium [(CF₂CF₂CF₂S₂O₄)N]⁻ nd >330 1,1,2,2,3,3-hexafluoropropane-1,3- disulfonyl-imide 44 3-(2-methoxyethyl)-1,2-dimethyl- imidazolium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]⁻ 431 >330 45 3-(2-methoxyethyl)-1,2-dimethyl- [(CF₃CF₂SO₂)₂N]⁻ 410 >330 imidazolium bis(pentafluoroethylsulfonyl) imide 46 2,3-dimethyl-1-propanolimidazolium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]— 436 >330 47 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]— 434 >330 48 1-propyl-3,5-dimethyl-pyridinium bis(trifluoromethylsulfonyl) imide

[(CF₃SO₂)₂N— 403 >330 49 1-(2-hydroxyethyl)-3,5-dimethyl- pyridinium bis(trifluoromethylsulfonyl)imide

396 >330 50 ethyldimethylpropylammonium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N— 408 >330 51 N-ethyl-2-hydroxy-N,N-dimethylethan- ammonium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N— 390 >330 52 N-ethyl-2-hydroxy-N,N- [(CF₃CF₂SO₂)₂N— ? >330 dimethylethaneammonium bis(pentafluoromethylsulfonyl)imide 53 N-(2-hydroxyethyl)-N,N- dimethylpropaneammonium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N— 388 >330 54 80%

[(CF₃SO₂)₂N]⁻ 439 >330 20%

[(CF₃CF₂SO₂)₂N]⁻ 55 80%

[(CF₃SO₂)₂N]⁻ 455 >330 20%

[(CF₃SO₂)₂N]⁻ 56 80%

[(CF₃SO₂)₂N]⁻ 455 >330 20%

[(CF₃CF₂SO₂)₂N]⁻ 57 80%

[(CF₃SO₂)₂N]⁻ 456 >330 20%

[(CF₃SO₂)₂N]⁻ 58 90%

[(CF₃SO₂)₂N]⁻ 456 >330 10%

[(CF₃SO₂)₂N]⁻ 59 ethyldimethylpropylammonium [(CF₃SO₂)₂N]⁻ 420 >330 bis(trifluoromethylsulfonyl)imide 60 1-propyl-4-methylpyridin-1-ium [(CF₃SO₂)₂N]⁻ 411 >330 bis(trifluoromethylsulfonyl)imide 61 1-ethyl-3-methylpyridin-1-ium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]⁻ nd >330 62 1-propyl-3,5-dimethylpyridinium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]⁻ 361 >330 63 1-(2-hydroxyethyl)-3,5- dimethylpyridinium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]⁻ 388 >330

Thus, the selection of the cations and of their specific substituents, as well as the selection of the anions and of their specific substituents make it possible to obtain an ionic liquid having a deposition start temperature, determined by MCT, higher than 330° C.

E. Examples of Ionic Liquids Having a Degradation Start Temperature Determined by MCT Higher Than 350° C. (Table 4)

TABLE 4 TGA MCT Ex Nomenclature Cation Anion (10° C./mn) (250-300° C.) 64 1-ethyl-2,3-dimethyl-imidazolium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]⁻ 434 >350 65 1,2-dimethyl-3-((trimethylsilyl)methyl)- imidazolium bis(trifluoromethylsulfonyl)imide

439 >350 66 1,2-dimethyl-3-((trimethylsilyl)methyl)- [(CF₃SO₂)₂N]— 412 >350 imidazolium bis(trifluoromethylsulfonyl) imide

Thus, as Table 4 above shows, a drastic selection of the cations and of the anions has made it possible to obtain ionic liquids having a deposition start temperature, determined by MCT, higher than 350° C.

F. Thermal Stability of Some Oils Available on the Market as Determined by MCT (Table 5)

As Table 5 here below shows, the conventional oils available on the market have a less good thermal stability and resistance to coking than the specific ionic liquids according to the invention.

TABLE 5 TDD for MCT Nomenclature 250-300(° C.) Turbonycoil 600 <250 Mobil Jet Oil II <250 Turbonycoil 640 294 BP Turbo Oil 2197 294

G. Comparison of the Thermal Stability Determined by TGA and MCT of Some Ionic Liquids Described in the Prior Art Patents (Table 6)

TABLE 6 TGA MCT Nomenclature Cation Anion (10° C./mn) 250-300° C. Ref. 1-ethyl-3-methylimidazolium methylsulfate

CH₃SO₄ ⁻ 350 <250 WO2011/026990A1 trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]⁻ 396 274 WO2011/026990A1 1-butyl-4-methylpyridinium hexafluorophosphate

BF₄ ⁻ 392 <250 US2010/0227785A1 1-butyl-4-methylpyridinium PF₆ ⁻ 341 <250 US2010/0227785A1 tetrafluoroborate 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]— 442 296 EP2602307 1-(2-methoxyethyl)-1- methylpyrrolidinium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]⁻ TDD very probably lower than 330° C. US2009/0069204 1-butyl-pryridinium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]⁻ TDD very probably lower than 330° C. US2009/0069204 N,N-diethyl-2-methoxy-N- methylethaneammonium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]⁻ Not very stable ionic liquid US2009/0069204 3-hexyl-1-methylimidazolium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]⁻ 421 254 US2009/0069204 3-butyl-1-methylimidazolium hexafluorophosphate

PF₆ ⁻ 338 <250 US2009/0069204 3-butyl-1-methylimidazolium BF₄ ⁻ 424 <250 US2009/0069204 tetrafluoroborate 3-ethyl-1-methylimidozalium bis(fluorosulfonyl)imide

[(FSO₂)₂N]⁻ 281 <250 US2013/0053287A1 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide

[(CF₃SO₂)₂N]⁻ TDD very probably lower than 330° C. US2013/0053287A1

Likewise, the ionic liquids described in the prior art mentioned above have a less good thermal stability and resistance to coking than the specific ionic liquids according to the invention. Thus, both ionic liquids disclosed in the patent application US 2010/0227785A1 do not have a particular resistance to thin-film coking.

It has also been noted that the ionic liquids described in the document WO2011/026990A1 have a very bad thin-film coking behavior; which clearly illustrates the difference between an additive reducing coking and a lubricant reducing deposits in thin film.

Moreover, as is represented in FIG. 2, which shows the result of the MCT test (250-300° C.) of 3-butyl-1-methylimidazolium hexafluorophosphate (TDD lower than 250° C.), carbonaceous deposits are visible. This compound therefore has a low resistance to coking. On the contrary, FIG. 3 illustrates the result of the MCT test (300-350° C.) of an ionic liquid according to the invention, 1,2-dimethyl-3-((trimethylsilyl)methyl)imidazolium bis(pentafluoromethylsulfonyl)imide (TDD >350° C.) where no deposit of carbonaceous residues is detected. This compound is thus highly resistant to coking. 

1-14. (canceled)
 15. Method of lubrication which comprises a step of lubricating a machine with a lubricating composition, the lubricating composition comprising, in mass with respect to the total mass of said lubricating composition, from 20% to 100% of an ionic liquid or of a mixture of several ionic liquids, wherein said ionic liquid or said mixture of ionic liquids is selected from an ionic liquid or a mixture of ionic liquids comprising at least: an anion A⁻ chosen from sulfonylimides, the substituent(s) of which is (are) independently chosen from a fluoroalkyl, fluoroether, perfluorinated alkyl or perfluoroether group, and a cation C⁺ comprising a nitrogen-containing heterocycle or a quaternary ammonium, the substituent(s) of which is (are) independently chosen from: a hydrogen atom or alkyl, alkoxy, fluorinated alkyl, perfluorinated alkyl, alkylsilane, alkyl alcohol, vinyl, alkyl allyl, ether or polyether groups having a linear or branched chain having from 1 to 3 carbon atoms, on condition that when the cation is a quaternary ammonium, at least two of these substituents are a methyl group, the deposition start temperature (TDD) in thin film, determined by the MCT method according to the standard GFC Lu-27 A-13, of the ionic liquid or of the mixture of ionic liquids being at least equal to 330° C., said lubricating composition being suitable to reduce the deposits formed in said machine.
 16. Method according to claim 15, in which the lubricating step is carried out at high temperatures of between 200 and 500° C.
 17. Method according to claim 15, in which said cation C⁺ comprising a nitrogen-containing heterocycle is selected among: imidazolium, pyrazolium, quinolium, pyridinium, piperidinium, oxazolium, thiazolium, benzothiazolium or morpholinium.
 18. Method according to claim 15, in which the cation C⁺ comprising a nitrogen-containing heterocycle is selected among: imidazolium, pyridinium, or pyrazolium.
 19. Method according to claim 15, in which the deposition start temperature (TDD) in thin film, determined by the method MCT according to the standard GFC Lu-27 A-13, of said ionic liquid or of said mixture of ionic liquids is at least equal to 350° C.
 20. Method according to claim 17, in which the cation C⁺ is imidazolium comprising at least two methyl groups at position 1 and 2 or at position 2 and 3, or a pyridinium comprising at least one methyl group.
 21. Method according to claim 20, in which the cation C⁺ is a pyridinium comprising at least one methyl group at position 3, 4 or
 5. 22. Method according to claim 20, in which the imidazolium comprising a methyl group at position 1 and 2 or at position 2 and 3 comprises a hydrogen atom at position 4 and
 5. 23. Method according to claim 22, in which the imidazolium cation C⁺ is 1-ethyl-2,3-dimethylimidazolium or 1,2-dimethyl-3-((trimethylsilyl)methyl)imidazolium.
 24. Method according to claim 22, in which the imidazolium, whose substituents in position 1 and 2 are methyls, the substituents in position 4 and 5 are hydrogen atoms, comprises a substituent in position 3 independently selected among the alkyl, fluorinated alkyl, perfluorinated alkyl, alkyl silane, alkyl alcohol or vinyl groups having a linear chain or a branched chain with 1 to 3 carbon atoms.
 25. Method according to claim 15, in which said

sulfonylimide corresponds to the following general formula: in which R₁ and R₂ are identical or different and independently selected among a fluoroalkyl, fluoroether, perfluorinated alkyl or perfluoroether group, such as [(CF₃SO₂)₂N]⁻, [(CF₃CF₂SO₂)₂N]⁻, [(CF₃CF₂CF₂CF₂SO₂)₂N]⁻ or [(CF₃CF₂CF₂SO₂)₂N]⁻.
 26. Method according to claim 25, in which said sulfonylimide corresponds to [(CF₃SO₂)₂N]⁻, [(CF₃CF₂SO₂)₂N]⁻, [(CF₃CF₂CF₂CF₂SO₂)₂N]⁻ or [(CF₃CF₂CF₂SO₂)₂N]⁻.
 27. Method according to claim 15, in which the ionic liquid or at least one of the ionic liquids of the mixture is selected among: 3-(2-hydroxyethyl)-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide; 3-(2-hydroxypropyl)-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide; 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide; 1-ethyl-2,3-dimethylimidazolium bis(pentafluoroethylsulfony)imide; 3-allyl-1,2-dimethylimidazolium bis(trifluoromethylsulfonyl)imide; 1,2-dimethyl-3-(trimethylsilyl)methyl)imidazolium bis(pentafluoroethylsulfony)imide; 1,2-dimethyl-3-((trimethylsilyl)methyl)imidazolium bis(trifluoromethylsulfonyl)imide; 1,2-dimethyl-3-((trimethylsilyl)propyl)imidazolium bis(trifluoromethylsulfonyl)imide; 2,3-dimethyl-1-propylimidazolium bis(pentafluoroethylsulfony)imide 2,3-dimethyl-1-propylimidazolium bis(trifluoromethylsulfonyl)imide; 2,3-dimethyl-1-propylimidazolium 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonylimide; 3-(2-methoxyethyl)-1,2-dimethylimidazolium bis(pentafluoroethylsulfony)imide; 3-(2-methoxyethyl)-1,2-dimethylimidazolium bis(trifluoromethylsulfonyl)imide; 2,3-dimethyl-1-propanolimidazolium bis(trifluoromethylsulfonyl)imide; 1,2 dimethyl-3-allylimidazolium bis(trifluoroethylsulfonyl)imide; 1-propyl-3,5-dimethylpyridinium bis(trifluoromethyl)sulfonyl)imide; 1-(2-hydroxyethyl)-3,5-dimethylpyridinium bis(trifluoromethyl)sulfonyl)imide; ethyldimethylpropylammonium bis(trifluoromethylsulfonyl)imide; N-ethyl-2-hydroxy-N,N-dimethylethaneammonium bis(trifluoromethylsulfonyl)imide; N-ethyl-2-hydroxy-N,N-dimethylethaneammonium bis(pentafluoroethylsulfonyl) imide; N-(2-hydroxyethyl)-N,N-dimethylpropaneammonium bis(trifluoromethylsulfonyl) imide; 1-propyl-4-methylpyridin-1-ium bis(trifluoromethylsulfonyl)imide; 1-ethyl-3-methylpyridin-1-ium bis(trifluoromethylsulfonyl)imide; 1-propyl-3,5-dimethylpyridinium bis(trifluoromethylsulfonyl)imide; 1-(2-hydroxyethyl)-3,5-dimethylpyridinium bis(trifluoromethylsulfonyl)imide; or a mixture thereof.
 28. Method according to claim 15, in which the lubricating composition comprises at least 50% to 100% of said ionic liquid with respect to the total mass of the lubricating composition.
 29. Method according to claim 28, in which the lubricating composition comprises at 75 mass-% to 100 of said ionic liquids with respect to the total mass of the lubricating composition.
 30. Method according to claim 15, in which the lubricating composition comprises from 0 mass-% to 25 mass-% of one or of several additives with respect to the total mass of the lubricating composition.
 31. Method according to claim 30, in which the additive or the additives are selected from: anti-wear agents, anti-corrosion agents, antioxidants, and a mixture of two or more of these additives.
 32. Method for reducing the deposits formed in a machine by using a lubricating composition, in which said lubricating composition comprises, by mass with respect to its total mass, from 20% to 100% of an ionic liquid or of a mixture of several ionic liquids, said ionic liquid or said mixture of ionic liquids being selected from an ionic liquid or a mixture of ionic liquids comprising at least: an anion A− chosen from sulfonylimides, the substituent(s) of which is (are) independently chosen from a fluoroalkyl, fluoroether, perfluorinated alkyl, perfluoroether or perfluorosulfonyl group, and a cation C+ comprising a nitrogen-containing heterocycle or a quaternary ammonium, the substituent(s) of which is (are) independently chosen from: a hydrogen atom or alkyl, alkoxy, fluorinated alkyl, perfluorinated alkyl, alkylsilane, alkyl alcohol, vinyl, alkyl allyl, ether or polyether groups having a linear or branched chain having from 1 to 3 carbon atoms, on condition that when the cation is a quaternary ammonium, at least two of these substituents are a methyl group, the deposition start temperature (TDD) in thin film, determined by the MCT method according to the standard GFC Lu-27 A-13, of the ionic liquid or of the mixture of ionic liquids is at least equal to 330° C.
 33. Method according to claim 32, wherein the deposition start temperature (TDD) in thin film, determined by the MCT method according to the standard GFC Lu-27 A-13, of the ionic liquid or of the mixture of ionic liquids is higher than or equal to 350° C.
 34. The method of claim 20, wherein the cation C⁺ is a pyridinium comprising at least one methyl group at position 3, 4 or
 5. 